As is well known, tuners between a transmitter and an antenna are used to match the antenna impedance to the impedance at the output of a transmitter. In amateur radio service, for instance, one is seeking to match antennas to transmitters in various bands, for instance, from 160 meters through 10 meters. When using a T network, while a variable input matching capacitor can be utilized for matching an antenna to the 50-ohm output of a transmitter at the low frequency bands, when trying to use the same circuit to tune 10-meter antennas, even when the variable input matching capacitor is completely open, there is nonetheless a minimum capacitance between the plates of the open variable capacitor. Because of the minimum capacitance, if for instance an antenna has an impedance of 1,000 ohms, its impedance may only be reduced to 100 ohms. There is thus a mismatch between the output impedance of the transmitter and that of the antenna that increases the standing wave ratio.
It has been found that shunting the tuner input to ground through a capacitor is effective in reducing the mismatch, for instance, from 100 ohms down to 50 ohms. However, if one is using a fixed shunt capacitor, the capacitor must have an extremely high current rating, for instance as high as 10 amps or more for continuous high power applications such as associated with AM and RTTY transmission. While 3 KV capacitors have been used in tuners and are satisfactory for low duty cycle use such as for high power CW and SSB transmissions, they oftentimes fail and burn up, especially when used with 1500-watt PEP linear amplifiers.
By way of further background, antenna tuners in the 1940s and 1950s used so-called link-coupled antenna tuners. However, in order to obtain appropriate matching range, these tuners involved clipping wires to the various components and moving jumpers around, making them inconvenient to use. One therefore had to actually change the configuration of the circuit to work with different kinds of antenna impedances.
Collins Radio in the 1930s and 1940s developed what has become known as their Pi network. The Pi network has an advantage because three controls are used to match most antennas to the output impedance of a transmitter. The Pi networks typically use two large variable capacitors and a variable inductor in which the variable inductor could either be a switched inductor or a continuously variable inductor.
One of the problems of the Pi network was that it took an exceedingly large amount of capacitance, namely several thousand picofarads of capacitance, in order to match certain antennas. Moreover, while the Pi network was very convenient, a single-ended Pi network did not work with balanced lines unless one used a balun. Even then, the result was non-optimal.
During this time and even prior to the development of the Pi network, L networks were used for antenna tuning. They were not particularly popular and were troublesome to use because in order to match a high impedance, one had to have one configuration, whereas if one wished to match a low impedance, one had to switch to a different configuration. One would then have to go through a tuning procedure each time the configuration was switched.
Moreover, the L networks required large amounts of capacitance, which could be as much as 5,000-picofarads. With smaller common air variable capacitors, one would nonetheless have to switch in many fixed capacitors. Moreover, if one had a variable capacitor that was as large as 5,000 picofarads, then one would have to slowly and carefully open that capacitor in order to fine-tune the match because of the large change in capacitance per degree of rotation. However, the minimum capacitance might be so high that it would not tune a wide range of impedances on the highest frequency band of interest.
In terms of amateur radio frequency bands, one wishes to be able to tune from approximately 1.8 MHz to 30 MHz. The problem is that if one makes an L network perform well on the low frequency bands, namely 160 meters, then one has problems matching antennas at the higher frequency bands, especially at 10 meters.
The reason for the difficulty in matching antennas at the high frequency end is because of the minimum capacitance of the variable capacitor. If one designs a variable capacitor to work well on 10 meters, then it will not work very well on 160 meters. This is because one needs large amounts of capacitance when tuning on 160 meters. One typically achieves such high capacitance by switching in additional capacitance with an expensive switch.
As to shunt capacitors, in the past, various systems have employed variable capacitors such as in the semi-T network antenna tuner described by Lewis G. McCoy in the Jul. 1970 issue of QST published by the American Radio Relay League. McCoy called his antenna tuner the Ultimate Transmatch, which employed two capacitors ganged together, with the input signal from the transmitter coupled to the junction between the two capacitors. One of the variable capacitors ran from the transmitter to ground, whereas the other variable capacitor ran from the transmitter to the ungrounded end of the inductor and in the semi-T network.
It is not clear what the McCoy shunt capacitor accomplished and it was dropped in later T-network tuners.
The variable shunt capacitor in the McCoy tuner did not, however, function to extend the matching range of the antenna tuner. The reason was simply that, as the variable input matching capacitor had its capacitance decreased, so did the capacitance associated with the shunt capacitance. This decreased capacitance did not address the minimum capacitance problem that limits the matching range of a T-type tuner.
Note that a commercial version of the McCoy tuner, namely the MURCH UT-2000, is described in QST, Dec. 1972, American Radio Relay League.
As has been stated above, the McCoy tuner has been referred to as a semi-T network antenna tuner. The semi-T network antenna tuner of McCoy worked relatively well and served the basis for the evolution of the modem T network that uses an inductor and two capacitors, a input matching capacitor and an output matching capacitor.
However, in the evolution of the T network, instead of having to provide 1,000 picofarads or even 500 picofarads of capacitance, it was found that one could use common 250-picofarad variable capacitors with a variable inductor and match an extremely wide range of impedances. With 250-picofarad variable capacitors, one can achieve exceptional performance on 10 meters while achieving adequate performance at 160 meters, for instance, at 1.8 MHz. The problem is the efficiency at 1.8 MHz, which is poor because one loses a great deal of power in the antenna tuner.
The reason that the performance is poor on 160 meters is that use of the tuner at 1.8 MHz forces a high current through the roller inductor as well as applying a large voltage across the roller and across the capacitor. This results in a lot of losses in the roller inductor. Also the large voltage applied across the matching capacitors tends to cause high voltage arcing in the capacitors.
More importantly, because the value of the variable capacitor is small, namely 250 picofarads, when one opens the capacitor up, the minimum capacitance is also small. This allows one to readily tune in the 10-meter band. However, at 1.8 MHz, because of the low capacitance the efficiency is so bad for low impedance loads that one could potentially lose half of the power in the antenna tuner.
One way to alleviate this problem at 160 meters is to use a very large value for the input matching capacitor so a very large value output matching capacitor can be used. If one uses a 500-picofarad or 1,000-picofarad variable capacitor, then the losses for low impedance loads at 1.8 MHz are minimized and one can readily match low to very low antenna impedances.
However, when one uses a 1,000-picofarad capacitor and opens it up, the minimum capacitance is now high. In fact, the capacitance is so high that one cannot tune a high-impedance load on 10 meters. Thus, by enabling the tuner to work at 160 meters, one has effectively eliminated its performance at 10 meters. In short, if one wants to have high efficiency at 160 meters, the tuner hardly works at all on 10 meters.
In order to extend the high-impedance matching range of a T network tuner, MFJ Enterprises, Inc. of Starkville, Miss. produced a tuner in which a fixed shunt capacitor was connected between the transmitter and ground at the input to the tuner to solve the problem of the high minimum capacitance at the higher frequencies. This capacitor was three 33-picofarad 3-KV high voltage capacitors in parallel. While this capacitor was suitable for some low duty cycle transmissions such as CW and SSB, for continuous use the currents through this capacitor were exceedingly high and could, for a 1.5-kilowatt amplifier, result in 10 amps or more of current through the fixed capacitor, which could destroy it.
Thus these fixed capacitors could burn up in continuous use and sometimes with heavy CW or SSB use. The result of such large amounts of current through the fixed capacitor would be that it would burn holes through the capacitor and the capacitor would catch fire. The reason that these fixed capacitors are not suitable for high-current usage is that the dielectric between the two plates can melt or burn.
Another problem with the fixed shunt capacitor occurs in the high frequency range when a low impedance load is to be matched. Here the fixed shunt capacitor reduces the effective capacitance of the input matching capacitor. In order for the input matching capacitor to perform it must increase its capacitance. However, the fixed shunt capacitor counters this.
Note that in T and L networks one can utilize a small shunt coil to put a reactance across the input to extend the low impedance matching range on 160 meters. The problem with such a solution is that while on 160 meters the coil could be effective, for other bands one would have to switch in different shunt coils.